JPH07254434A - Nonaqueous electrolyte for lithium battery - Google Patents

Abstract

PURPOSE:To provide electrolyte of high electric conductivity, and achieve excellent cycle characteristics, and properties such as large capacity as it is assembled in a battery by using Li(CF3SO2)2N as the electrolyte. CONSTITUTION:An Li battery comprises a positive electrode comprising active material of metal oxide or sulfate, carbon material which is capable of storing/ discharging metal Li, Li alloy or Li ion, and nonaqueous electrolyte, where 1-2 sort(s) of ethylene carbonate, propylene carbonate, and theta-lactone selected as high dielectric constant solvent is mixed with l-2 sort(s) of dimethyl carbonate. ethylmethyl carbonate, and fatty acid ester selected as low viscosity solvent to dissolve Li(CF3SO2)2N as electrolyte. A mixing ratio of Li(CF3SO2)2N to the nonaqueous solvent is 0.5-2.5mol/l.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte used in a lithium primary or secondary battery, and more specifically, a positive electrode using a metal oxide or a sulfide as an active material and a metal. The present invention relates to a non-aqueous electrolytic solution used in a lithium battery including a negative electrode made of a carbonaceous material capable of inserting and extracting lithium or a lithium alloy or lithium ions, and a non-aqueous electrolytic solution.

[0002]

2. Description of the Related Art Lithium primary or secondary batteries using a non-aqueous electrolyte have high battery voltage characteristics and are suitable as a power source for various electronic devices which have undergone remarkable development in recent years as high energy density batteries. Is.

Solvents used in the non-aqueous electrolyte of this type of battery include propylene carbonate (PC), γ-butyrolactone (γ-BL) and diethyl carbonate (D
EC), 1,2-dimethoxyethane (DME) and 2
-Methyltetrahydrofuran (2-MeTHF) and the like are used alone or in combination of two or more. LiClO ₄ , LiPF ₆ , and LiBF ₄ are used as the electrolyte, but LiClO ₄ is generally used because it has high electric conductivity and various excellent characteristics.

[0004]

However, this L
Secondary batteries using iClO ₄ as an electrolyte have the drawback that when used in severe environments such as high-temperature discharge, overcharge, and short circuit, the performance tends to deteriorate and the internal pressure rises, and there is a risk of explosion. was there.

It is also known that when a battery using this electrolytic solution is charged to a high potential, the electrolytic solution undergoes oxidative decomposition, and this oxidation adversely affects the capacity, cycle characteristics and safety of the battery. become.

Therefore, the inventors of the present invention searched various electrolytes having an electric conductivity equal to or higher than that of LiClO ₄ and which can be used under stable conditions, and found that Li (CF ₃ SO 4
₂ ) Focused on ₂ N.

This electrolyte Li (CF ₃ SO ₂ ) ₂ N is
Dissolves in non-aqueous solvent and LiCl in non-aqueous solvent
It was confirmed by the present inventors that the substance has a high electric conductivity like O ₄ and has an extremely stable property.

The present invention was made on the basis of the above findings, and its purpose is to use Li as an electrolyte.
By using (CF ₃ SO ₂ ) ₂ N, it has high electrical conductivity and excellent stability such as resistance to oxidative decomposition at high potential, and has excellent cycle characteristics and high capacity in a state of being incorporated in a battery. It is intended to provide a non-aqueous electrolytic solution for a lithium battery which can bring out such characteristics.

[0009]

In order to achieve the above object, the present invention provides a positive electrode using a metal oxide or a sulfide as an active material, and a metal lithium or a lithium alloy or a carbon capable of inserting and extracting lithium ions. In a lithium battery including a negative electrode made of a high-quality material and a non-aqueous electrolyte, the non-aqueous electrolyte is ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL) as a high dielectric constant solvent. 1 to 2 types, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fatty acid ester as low viscosity solvent
It is characterized in that Li (CF ₃ SO ₂ ) ₂ N is dissolved as an electrolyte in a solvent in which two kinds are selected and mixed.

As the fatty acid ester, R ₁ --CO
Indicated by O-R _2, R ₁ and R ₂ are C _n H _{2n + 1}
Is an alkyl group of n = 1-5, preferably n = 1-3. Examples thereof include ethyl acetate, propyl acetate, isopropyl acetate, methyl propionate and methyl butyrate.

Further, the reason why the above-mentioned high dielectric constant solvent and low viscosity solvent are mixed and used as the solvent is generally that when the component ratio of the high dielectric constant solvent is large, the battery characteristics become insufficient due to an increase in viscosity at low temperature. On the contrary, if the component ratio of the low-viscosity solvent is high, the solubility of the electrolyte is lowered, and as a result, the electrical conductivity is lowered, which causes inconvenience.

Preferably, the Li (CF ₃ SO
₂ ) ₂ N is 0.5 to 2.5 mo with respect to the non-aqueous solvent.
l / l mixing. The reason for this limitation is
When it is less than 0.5 mol / l, the battery does not have sufficient electric conductivity, and when it is more than 2.5 mol / l, it does not dissolve in a non-aqueous solvent, or when it is at a low temperature, crystal precipitation occurs. This is because such problems occur, and a more preferable range is 1 to 1.5 mol / l.

When PC is used as the high dielectric constant solvent, it is desirable to add PC in an amount not exceeding the amount of EC. In the case of a lithium battery using graphite for the negative electrode, it has been conventionally known that excessive PC causes a decrease in battery performance, and therefore, the above restrictions are imposed.

[0014]

ACTION AND EFFECT The non-aqueous electrolyte solution of the present invention using Li (CF ₃ SO ₂ ) ₂ N has a higher potential than the conventional non-aqueous electrolyte solution using LiClO ₄ and has oxidation decomposition resistance characteristics. , A lithium secondary battery incorporating this has little capacity deterioration during charging and can be charged to a high potential.

That is, in a conventional lithium secondary battery using a non-aqueous electrolytic solution containing LiClO ₄ as an electrolyte, an irreversible increase in current occurs at a relatively low potential during charging, and the electrolytic solution is increased in response to this increase. Although it is known to oxidize, the lithium secondary battery using the non-aqueous electrolytic solution of the present invention does not have an irreversible increase in current up to a high potential and can be used as a stable electrolytic solution.

Therefore, there is an advantage that the cutoff voltage at the time of charging can be set high and a large current can be taken out by one charging.

Further, the oxidation-decomposition resistance does not cause deterioration even after repeated charging and discharging, so that the charging and discharging cycle characteristics can be remarkably improved and the safety can be greatly improved.

The non-aqueous electrolyte containing the above electrolyte is
As a matter of course, it can be applied to a lithium primary battery as well as a lithium secondary battery.

[0019]

EXAMPLES Next, examples of the present invention will be described. However, the present invention is not limited to the examples.

Example 1 1 to 2 kinds of EC, PC and γ-BL as a high dielectric constant solvent and 1 to 2 kinds of DMC, EMC and propyl acetate as a low viscosity solvent, respectively. 1 mol / l of Li (CF
Cyclic voltammetry was performed using electrolyte solutions A to T mixed with ₃ SO ₂ ) ₂ N. In addition, the working electrode is L
A mixture of iCoO ₂ with carbon powder as a conductive agent and Teflon powder as a binder at a weight ratio of 100: 10: 6, metal lithium was used as the counter electrode and the reference electrode, and the scanning speed was 40 mv / h. In the comparative example, PC / LiClO ₄ which is a standard composition of the conventional electrolytic solution is used.
(1 mol) (indicated by U in Table 1) was used.

[0021]

[Table 1] This result shows that Comparative Example U
It can be seen that in about 4.5 V, an irreversible increase in current occurs, and the electrolytic solution begins to oxidize. In comparison with this, the oxidation of Examples A to T starts at about 5V.

Therefore, since the electrolytes A to T are stable up to a high potential, the cutoff voltage at the time of charging can be set high, a large current can be taken out as a battery, and the safety at the time of charging is greatly improved. It turned out that it can be improved.

Example 2 Next, in order to examine the actual battery performance, the test cells shown in FIG. 2 were assembled using the electrolytic solutions A to T of the present invention having the compositions shown in Table 1 above.

In the figure, LiCoO ₂ , carbon powder as a conductive agent, and Teflon powder as a binder are mixed at a weight ratio of 100: 10: 6 and rolled to form a positive electrode mixture 1 formed into a sheet, A Ti net serving as the current collector 2 was pressure-bonded to obtain a positive electrode. Also, a carbonaceous powder obtained by firing pitch-based carbon fiber and EPDM (ethylene propylene diene monomer) as a binder were mixed in a ratio of 100: 7 and rolled to form a negative electrode mixture. 3 was pressure-bonded to a Ni net as a current collector 4 to obtain a negative electrode.

Both the positive electrode mixture 1 and the negative electrode mixture 3 have a square shape of 10 × 10 mm in plan view, and the thickness of the positive electrode mixture 1 is 0.
The thickness of the negative electrode mixture 3 is 25 mm, and the thickness of the negative electrode mixture 3 is 0.40 mm. Also,
The gap between the two mixtures 1 and 3 was set to 2 mm, and they were arranged in the beaker 5 as shown. As the electrolytic solution, in addition to the electrolytic solutions having the compositions A to T, one kind of conventional electrolytic solution U as a comparative example was evaluated under the same conditions.

The theoretical charge capacity of the positive electrode is 8.2 mA.
h, the theoretical charge capacity of the negative electrode is 7 mAh, and the reason why the theoretical charge capacity of the positive electrode is larger than that of the negative electrode is that the amount of lithium that can be involved in the subsequent discharge after the first charge is smaller than the charge capacity. is there. This is because the charged lithium is taken into the carbonaceous negative electrode in a fixed amount only in the first charge / discharge cycle and cannot be discharged from the next time.

Next, 300 cycles of charge / discharge cycle characteristic test in which constant current charging / discharging with a charging current of 1 mA and a discharging current of 2 mA was performed on each of the test cells having the above specifications using the electrolytic solutions A to T and the electrolytic solution U as a comparative example. Carried out.
The discharge end voltage was set to 2.8 V, and the charge voltage was set to 4.15 V as an upper limit in consideration of the characteristics of the conventional electrolyte.

The results are shown in FIGS. 3 (a) to 3 (d) and Table 2. In this figure, the initial discharge capacity is 100, and 50,1
The discharge capacity ratio (%) in the cycles of 00, 150, 200, 250, and 300 is shown. As is clear from FIG. 3 and Table 2, when the electrolytic solutions A to T of the present invention are used, the cycle is 300 cycles. It is about 90% higher than the first time. On the other hand, in the comparative example, the cycle characteristic was 69.7% with respect to the initial value after 300 cycles, and it was confirmed that the cycle characteristics were significantly improved in the present invention compared with the conventional case.

[0029]

[Table 2] Reference Example 1 Before carrying out Examples 1 and 2 above, Li (CF ₃ SO ₂ ) ₂
In order to find the optimum combination and mixing ratio of the mixed solvent for dissolving N, 1M was added to the solvent of each combination and mixing ratio.
_{2 when} Li (CF ₃ SO ₂ ) ₂ N is dissolved
The electrical conductivity was measured at 5 ° C. This result is shown in Figure 4.
6A and 6B are shown in FIGS. Table 3 shows the optimum combination of solvents and the mixing range derived from this figure.

[0030]

[Table 3] If the ratio of the mixing ratio shown in Table 3 is exceeded, first, if the ratio of the high dielectric constant solvent is large, the battery characteristics at low temperature are not sufficient, and conversely, if the ratio of the low viscosity solvent is large, the electrolyte is Since the solubility of the solvent decreases and the electric conductivity decreases, and as a result, good battery characteristics cannot be obtained, the combination range of the mixed solvent is limited to the mixing range shown in Table 3 above. .

Reference Example 2 Prior to carrying out Examples 1 and 2 above, L of the non-aqueous electrolytic solution of the present invention was used.
In order to search for the optimum concentration of i (CF ₃ SO ₂ ) ₂ N, as a typical example, EC + DMC (1: 1), BL + EC (1:
1), EC + DMC + PrAc (propyl acetate) (3:
5: 2) Li (CF ₃ ) in which the concentration was changed to the mixed solvent of the mixing ratio
The electrical conductivity at 25 ° C. when SO ₂ ) ₂ N was dissolved was measured. The results are shown in FIGS. 7 (a) to 7 (c).

As is clear from the results shown in FIG. 5, Li (C
The concentration of F ₃ SO ₂ ) ₂ N is preferably in the range of about 0.5 to 2.5 mol / l. When the concentration is less than this range, the electrical conductivity is reduced due to the absolute amount of electrolyte being insufficient, and conversely, when it is large, the electrolyte is Can not be dissolved, rather lowering the electrical conductivity and causing problems such as electrolyte precipitation at low temperatures. Therefore, the above range is preferable, and 1.0 to 1.5 is more preferable.
It is understood that the optimum range is about mol / l.

[Brief description of drawings]

1A to 1D are graphs comparing cyclic voltammetry in Example 1. FIG.

FIG. 2 is a schematic diagram of a test cell in Example 2.

3 (a) to (d) are graphs comparing charge / discharge cycles in Example 2. FIG.

4 (a) and (b) are graphs showing the relationship between the mixing ratio of mixed solvents and electric conductivity in Reference Example 1. FIG.

5 (a) and (b) are graphs showing the relationship between the mixing ratio of mixed solvents and electric conductivity in Reference Example 1. FIG.

6 (a) and 6 (b) are graphs showing the relationship between the mixing ratio of mixed solvents and electric conductivity in Reference Example 1. FIG.

7 (a) to 7 (c) are graphs showing the relationship between electrolyte concentration and electric conductivity in Reference Example 2.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nozomu Narita 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd.

Claims (3)

[Claims] 1. A positive electrode using a metal oxide, a sulfide, or the like as an active material, a negative electrode made of metallic lithium, a lithium alloy, or a carbonaceous material capable of inserting and extracting lithium ions, and a nonaqueous electrolytic solution. In the lithium battery, the non-aqueous electrolytic solution contains 1-2 kinds of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL) as a high-dielectric constant solvent, and dimethyl carbonate ( 1 to 1 out of DMC), ethyl methyl carbonate (EMC), and fatty acid ester
A non-aqueous electrolyte solution for a lithium battery, wherein Li (CF ₃ SO ₂ ) ₂ N is dissolved as an electrolyte in a solvent in which two kinds are selected and mixed. 2. The lithium battery according to claim 1, wherein the Li (CF ₃ SO ₂ ) ₂ N is mixed in an amount of 0.5 to 2.5 mol / l with respect to the non-aqueous solvent. Non-aqueous electrolyte. 3. The non-aqueous electrolyte for a lithium battery according to claim 1, wherein PC is added to EC of the high dielectric constant solvent in an amount not exceeding EC.